The invention relates to a switched-mode power supply.
A number of problems occur particularly in the case of flyback converter-type power supplies which do not have their own auxiliary power supply for supplying the regulating circuit arranged in the switched-mode power supply with the necessary supply and regulating voltage for the running-up phase after a switch-on, thus, for example, due to the fact that the output voltage does not rise linearly, that the running-up time of the output voltage cannot be calculated, in practice, and that the running-up curve of the output voltage depends on the complex load resistance.
Since the regulating circuit is only supplied with the full supply and regulating voltage from the auxiliary winding after the running-up phase, a capacitor is needed for the regulating circuit which supplies the voltages needed by the regulating circuit for operating and starting the regulating process in this phase. At the beginning of the running-up phase, this capacitor must be capable of accommodating such an amount of energy that during the running-up phase, the supply voltage at the regulating circuit does not drop below the value at which the regulating circuit stops operating because of undervoltage and breaks off the running-up. The problem of sufficiently high energy storage, however, cannot be easily solved by means of a correspondingly large capacitor since otherwise the running-up attempt in the case of a short circuit at the output of the switched mode power supply would take too long, as a result of which components would possibly be overloaded for too long a period.
Another problem associated with the size of this capacitor is the delay time after which the regulating circuit carries out its first regulating attempt when the alternating line voltage is applied. This is determined by the capacitance of the capacitor concerned and its associated series resistor. The larger the capacitor, the longer it takes until the regulating circuit begins to regulate for the first time.
Since the output voltage has not yet reached its final value in the running-up phase, the energy balance in the transformer is not yet correct in this phase. In the ideal case, the energy balance should look as follows: EQU Ui * te=u * Ua * ta ##EQU1## EQU T=te+ta (2)
Where:
Ui - rectified switched-mode power supply input voltage PA1 Ua - output voltage of the switched-mode power supply PA1 u - transformation ratio of the number of turns (Nprim/Nsec) PA1 te - turn-on time of the switching transistor PA1 ta - turn-off time of the switching transistor PA1 T - duration of period
A switching transistor controlled by the regulating circuit therefore in each case switches to a transformer which is partially magnetized to different extents. The consequence of this is that the collector current of the switching transistor continues to rise and the transformer continues to be magnetized until the collector current is switched off when a maximum value is reached. It can happen that the collector current has already reached its maximum value shortly before the turn-on of the switching transistor and the regulating circuit immediately switches it off again. In the case of unfavorable circuit tolerances but mainly in the case of high switching frequencies and when bipolar switching transistors having relatively long storage times are used, considerable overloading can occur in the components since energy is still transported into the transformer during the storage times of the switching transistor.
If the complex load resistance at the output of the switched-mode power supply has such an effect that the output voltage does not rise fast enough, this may also lead to the transformer being magnetized up into saturation, as a result of which components can be destroyed.
The situation described above also affects an output diode in the secondary circuit of the switched-mode power supply. During the blocking phase of the switched-mode power supply, the output diode is switched to be conductive for as long as energy is stored in the transformer. The more energy is stored in the transformer, the greater the output diode current. During the running up phase, the output diode current has not yet dropped back to zero at the beginning of the conducting phase of the switched-mode power supply. This means that the output diode is suddenly switched from the forward direction to the reverse direction, which leads to additional heating of the component. This component, if it is not greatly overdimensioned, can rapidly heat up to such an extent that it is destroyed, especially in the case of switched-mode power supplies with high output voltages.
From documents EP,A,0 301 386, 01.02.1989 and IBM Technical Disclosure Bulletin, Vol. 31 No. 5, Oct. 5, 1988, pates 424-425, switched-mode power supplies known which deal with the running-up phase to such an extent that they contain measures by means of which the time for the running-up phase is shortened. However, this does not affect the variation of the running-up phase itself which should be as uniform as possible. The running-up phase is accelerated by deriving the supply voltage of the regulating circuit from the rectified line voltage via a serial timing section having as short a time constant as possible during the switching-on phase and only later from the sensing circuit. To avoid power loss, the serial timing section is disconnected after the running-up phase. The circuit complexity for these measures is correspondingly great.